1. Field of the Invention
This invention relates to a charged particle microscope such as an electron microscope equipped with an optical microscope, a laser scattering microscope or an optical height detection system.
2. Description of the Related Art
In a charged particle microscope such as an electron microscope or an ion microscope, a sample is generally placed on a multi-axes stage for achieving X-Y-Z three-dimensional driving for positioning relative to the microscope, rotation driving, inclination driving, and so forth, in order to observe a specific portion of the sample from a desired angle, to process the sample and to observe the sample while its position and posture can be freely changed. In the electron microscope, an optical microscope or a laser scattering microscope is additionally installed (1) to index a reference height of a sample surface, (2) to observe a surface in an optical image and (3) to conduct position indexing observation of very fine cracks and foreign matters, and the sample surface is observed separately from the observation operation by the charged particle microscope. In the electron microscope equipped additionally with the optical microscope or the laser scattering microscope, some of components and devices used for the electron microscope are not so designed as to cope with use in vacuum or cannot be put into vacuum for the reason of their materials. Because all of these members and devices cannot be arranged inside the sample chamber that must be kept in vacuum, observation is conducted while they are fitted to fixed portions such as sidewalls of the chamber.
FIG. 3 shows an example of an electron microscope that employs such a construction. In the drawing, reference numeral 1 denotes a body tube of an electron microscope. Reference numeral 2 denotes a vacuum chamber. Reference numeral 3 denotes a sample stage. Reference numeral 4 denotes a rotation mechanism. Reference numeral 5 denotes a Z-axis driving mechanism. Reference numeral 6 denotes a Y-axis driving mechanism. Reference numeral 7 denotes an X-axis driving mechanism. Reference numeral 8 denotes an inclination mechanism that is supported at an eccentric position of a rotary shaft mechanism 9 fitted to a sidewall of the vacuum chamber 2. The system further includes an optical microscope 10 for observing a sample surface separately from the electron microscope, fitted to a sidewall of the chamber 2. Reference numeral 11 in the drawing denotes a CCD camera for converting an image of the optical microscope 10 to electronic image information. FIG. 4 shows an example of a system including a laser level meter for indexing a reference height of the sample surface, provided to the electron microscope. In the same way as the example shown in FIG. 3, reference numeral 1 denotes a body tube of an electron microscope, reference numeral 2 denotes a vacuum chamber, reference numeral 3 denotes a sample stage and reference numeral 4 denotes a rotation mechanism. Although not shown in FIG. 4, the system also includes a Z-axis driving mechanism, a Y-axis driving mechanism, an X-axis driving mechanism, and an inclination mechanism in the same way as the example in FIG. 3. However, the system shown in FIG. 4 is different from the system shown in FIG. 3 in that a laser projector 12 and a laser receiver 13 serving as a laser level meter are fitted to the chamber 2 instead of the optical microscope shown in FIG. 3.
When the system described above is used for conducting foreign matter adhesion inspection of a semiconductor wafer, for example, the sample stage 3 is sometimes inclined so as to observe the sample from slantingly above and to classify defects. When this inclination driving of the sample stage 3 is made, it is necessary to prevent the position of the defect to be observed from deviating and coming off from the visual field of the microscope due to the driving operation. The center of the observation point by the microscope exists at the point of intersection between the optical axis of the microscope and the sample surface. In the related art example described above, the rotary shaft of the rotary shaft mechanism 9 supporting the inclination mechanism 8 is set so that its level is coincident with the sample surface lest the center position varies with the inclination driving operation. According to this construction, the crossing point of the surface with the optical axis of the electro-optical system does not move even when the inclination mechanism 8 changes the inclination of the sample surface. In the example shown in FIG. 3, however, the optical microscope 10 is fixedly installed to the chamber 2. Consequently, the sample surface inclines relative to this optical microscope 10, too and the problems arise in that (1) observation becomes impossible, (2) the observation point changes, (3) the observation condition changes, and so forth. In the example shown in FIG. 4, the measurement position of the laser level meter does not change but the direction of height changes due to inclination of the sample surface, affecting adversely the level measurement.